"Not at all. There is a very simple explanation, Harry..... This must have belong to Hans" will not be considered a leap of logic.
Step-by-step explanation:
This was said by Lidenbrock in jest since he was comparing the Hans size with others. Lidenbrock has no idea on Hans since they met only on the idea and he wanted to mock him for replacing his son.
So there is no way that it would be logically correct for Lidenbrock to speak about Hans to Harry. While Harry and Axel laughs at the joke, Hans show slight anger at Lidenbrock and calls him an Old Man.